An electric power steering apparatus which provides a steering mechanism of a vehicle with a steering assist torque (an assist torque) by means of a rotational torque of a motor, applies a driving force of the motor as the steering assist torque to a steering shaft or a rack shaft by means of a transmission mechanism such as gears or a belt through a reduction mechanism. In order to accurately generate the steering assist torque, such a conventional electric power steering apparatus performs a feedback control of a motor current. The feedback control adjusts a voltage supplied to the motor so that a difference between a current command value and a detected motor current value becomes small, and the adjustment of the voltage supplied to the motor is generally performed by an adjustment of a duty ratio of pulse width modulation (PWM) control.
A general configuration of the conventional electric power steering apparatus will be described with reference to FIG. 1. As shown in FIG. 1, a column shaft (a steering shaft, a handle shaft) 2 connected to a steering wheel 1 is connected to steered wheels 8L and 8R through reduction gears 3, universal joints 4a and 4b, a rack and pinion mechanism 5, and tie rods 6a and 6b, further via hub units 7a and 7b. Further, the column shaft 2 is provided with a torque sensor 10 for detecting a steering torque of the steering wheel 1 and a steering angle sensor 14 for detecting a steering angel θ, and a motor 20 for assisting the steering force of the steering wheel 1 is connected to the column shaft 2 through the reduction gears 3. Electric power is supplied to a control unit (ECU) 30 for controlling the electric power steering apparatus from a battery 13, and an ignition key signal is inputted into the control unit 30 through an ignition key 11. The control unit 30 calculates a current command value of an assist control based on a steering torque Tr detected by the torque sensor 10 and a vehicle speed Vel detected by a vehicle speed sensor 12, and controls a current supplied to the motor 20 for EPS based on a voltage control command value Vref obtained by performing compensation and so on with respect to the current command value.
Further, the steering angle sensor 14 is not indispensable and may not be provided, and it is possible to obtain the steering angle from a rotation sensor such as a resolver connected to the motor 20.
A controller area network (CAN) 40 exchanging various information of a vehicle is connected to the control unit 30, and it is also possible to receive the vehicle speed Vel from the CAN 40. Further, it is also possible to connect a non-CAN 41 exchanging a communication, analog/digital signals, a radio wave or the like except the CAN 40 to the control unit 30.
The control unit 30 mainly comprises a CPU (including an MPU and an MCU), and general functions performed by programs within the CPU are shown in FIG. 2.
Functions and operations of the control unit 30 will be described with reference to FIG. 2. As shown in FIG. 2, the steering torque Tr detected by the torque sensor 10 and the vehicle speed Vel from the vehicle speed sensor 12 are inputted into a current command value calculating section 101, and a current command value Iref0 is calculated by means of an assist map. The calculated current command value Iref0 is inputted into a maximum output limiting section 102 and an output is limited based on an overheat protection condition or the like in the maximum output limiting section 102. A current command value Iref whose maximum output is limited, is inputted into a subtracting section 103. Hereinafter, a section comprised of the current command value calculating section 101 and the maximum output limiting section 102 is referred to as a current command value determining section 108.
Moreover, with respect to the calculation of the current command value Iref0 performed in the current command value calculating section 101, it is also possible to calculate the current command value Iref0 by using not only the steering torque Tr and the vehicle speed Vel but also the steering angel θ.
The subtraction section 103 calculates a deviation ΔI (=Iref−Im) between the current command value Iref and a motor current Im of the motor 20 that is fed back, the deviation ΔI is controlled by a current control section 104 such as a proportional and integral (PI) control or the like, the controlled voltage control command value Vref is inputted into a pulse width modulation (PWM) control section 105, where the duty ratio is calculated, and in accordance with a PWM signal PS of which the duty ratio is calculated, the motor 20 is driven through a motor driving circuit 106. The motor current Im applied to the motor 20 is detected by a motor current detecting circuit 107, and the detected motor current Im is inputted into the subtracting section 103 to be fed back.
A bridge circuit that bridge-connects semiconductor switching elements (FETs) and the motor, is used in the motor driving circuit 106 that controls the motor current by means of the voltage control command value Vref and drives the motor 20. The motor driving circuit (inverter) 106 that is configured so as to control the motor current by ON/OFF-controlling the semiconductor switching elements in accordance with the duty ratio of the PWM signal determined based on the voltage control command value Vref, is used.
Generally, since the electric power steering apparatus is a so-called human-machine interface mechanism that directly and easily transmits the feel to a steering operator among automobile parts, the torque ripple caused by the motor and mechanical mechanism is took up as a problem of steering feeling performance. In particular, since floor vibration that is caused by the torque ripple caused by the motor and mechanical mechanism and occurs due to vehicle eigenvalue excitation also relates to the problem of a vehicle system's operating noise, it becomes a major problem. When friction in the reduction gears is small, a damping effect that is exerted when the friction is large is not expected, and an influence of vibration caused by the torque ripple or the like becomes larger.
However, since factors of the torque ripple range widely, if taking measures against torque ripple according to factor, there is a problem that it is not efficient.
As solutions for solving such a problem, for example, there are Japanese Unexamined Patent Publication No. S60-161257A (Patent Document 1), Japanese Unexamined Patent Publication No. 2006-188183 A (Patent Document 2), Japanese Unexamined Patent Publication No. 2009-090953 A (Patent Document 3), and WO 2009/078074 (Patent Document 4).
A vibration extraction method of “vehicle motion control apparatus” disclosed in Patent Document 1 is a configuration that extracts a vibration component having a specific (an arbitrary) frequency range based on a value (a steering angle) of a sensor for detecting vehicle behaviors by means of Fourier transform and suppresses vibrations by changing control parameters depending on the extracted vibration component having the arbitrary frequency range.
However, since a configuration for realizing Fourier transform is especially complicated, as a result, making heavy use of microcomputer resource, hence it is difficult to say that the vibration extraction method of Patent Document 1 is an efficient method. Further, in changing the control parameters, there is a problem that which parameter should be changed and the confirmation of trade-off matter with other performances tends to become very complicated.
A vibration extraction method of “electric power steering apparatus” disclosed in Patent Document 2 is a configuration that extracts a vibration component having an arbitrary intended frequency range based on a difference between a vibration center value calculated by performing a moving average with respect to a steering torque and a value obtained by extracting a specific vibration frequency by a band-pass filter with respect to the steering torque and suppresses vibrations by changing control parameters depending on the extracted vibration component having the arbitrary frequency range.
However, also in the vibration extraction method of Patent Document 2, in changing the control parameters, there is a problem that which parameter should be changed and the confirmation of trade-off matter with other performances tends to become very complicated.
A vibration extraction method of “electric power steering apparatus” disclosed in Patent Document 3 is a configuration that extracts a specific frequency component (1416 Hz) corresponding to a steering system's vibration caused by the application of an inverse input stress by performing a band-pass filter process and a root mean square (RMS) calculation with respect to a steering angle (a pinion angle), obtains an effective value of the extracted frequency component, and changes control parameters depending on a value (a power spectrum) after performing a low-pass filter process with respect to the obtained effective value.
However, since the electric power steering apparatus can not specify a steering pattern of a steering operator, in the case that a steer-inputted steering frequency is synchronized with the above specific frequency range (14˜16 Hz), there is a possibility that a steering component (the steering frequency) inputted by the steering operator with the intention exists in the above specific frequency range. Therefore, according to the vibration extraction method of Patent Document 3, at the time of such a steering pattern, a problem that the compensation works in a direction that blocks that steering pattern and there is a possibility of being accompanied by uncomfortable feeling occurs.
Further, as with Patent Document 1, also in the vibration extraction method of Patent Document 3, in changing the control parameters, there is a problem that which parameter should be changed and the confirmation of trade-off matter with other performances tends to become very complicated.
A vibration extraction method of “electric power steering apparatus” disclosed in Patent Document 4 is a method of utilizing a matter that amplitudes of the vibration components such as the torque ripple and a road surface disturbance are smaller than the amplitude of the steering component of the steering operator, and concretely is a configuration that extracts a vibration component (a small vibration component) having an arbitrary amplitude based on a difference between an output obtained by performing a hysteresis function process having a hysteresis width corresponding to the vibration component having the arbitrary amplitude with respect to a dynamic state parameter (a motor's rotational velocity or a steering torque) of an electric power steering apparatus or an automobile and the above dynamic state parameter, and calculates a vibration compensation value (a vibration suppression current) depending on the extracted vibration component to configure a feedback control loop.